Fine-grained 3C-SiC thick films prepared via hybrid laser chemical vapor deposition

An ultraviolet laser (λ = 266 nm) operated in pulsed mode and a diode laser (λ = 1060 nm) operated in continuous mode were simultaneously applied to create a hybrid laser chemical vapor deposition (CVD) approach. Fine-grained 3C-SiC thick films were prepared via hybrid laser CVD by using SiCl₄, CH₄ and H₂ as precursors. The effects of the ultraviolet laser on the preferred orientations, microstructures, microhardness values and deposition rates of 3C-SiC thick films were investigated. The 3C-SiC thick films that were prepared at 4 kPa via diode laser CVD exhibited <110>-orientations and 5-100 µm grain sizes, whereas those prepared via hybrid laser CVD were randomly oriented with 0.5-5 µm grain sizes. Compared to diode laser CVD, the additional irradiation of the ultraviolet laser in the hybrid laser CVD improved the Vickers microhardness values of the 3C-SiC thick films from 30 to 35 GPa, and the maximum deposition rate was also increased from 935 to 1230 µm/h.     

Fabrication of functional zirconia surfaces using a two-beam interference setup employing a picosecond laser system with 532-nm wavelength: Morphology,microstructure, and wettability

The aim of this work was to evaluate the feasibility of the fabrication of microtextures in zirconia using the direct laser interference patterning (DLIP) technique. A green ultra-short pulsed laser (532 nm, 10 ps) with a two-beam interference setup was used to produce line-like structures with a spatial period of 3 µm. For a fixed set of fluence and pulse-to-pulse overlap values (6.2 J/cm², 81%), periodic structures were successfully created for different hatch distances. The average depth of the features ranged from 0.37 µm for a hatch distance of 14.4 µm up to 0.84 µm for a hatch distance of 12.4 µm. However, a further decrease in hatch distance did not result in an increase in depth since the region of the ridges is also ablated. Scanning electron microscopy analysis showed pores formation on the laser grooves, but no sign of micro-cracking could be observed. Wettability tests showed an increase in hydrophobicity after DLIP. These results bring exciting perspectives on the fabrication of micro-textures with DLIP on zirconia surface.     

Nanosecond pulsed laser surface modification of yttria doped zirconia for Solid Oxide Fuel Cell applications: Damage and microstructural changes

An effective strategy to reduce the cathode polarization in a Solid Oxide Fuel Cell (SOFC) is to enlarge the cathode-electrolyte interface, corrugating the electrolyte surface of zirconia doped with 8 mol% of yttria (8YSZ) by pulsed-laser machining. However, laser-material interaction using a nanosecond pulsed laser can involve thermal effects on the surface. The objective of this work was to analyze the microstructural and phase changes, and the collateral damage caused by laser machining on the 8YSZ electrolyte surface of the SOFCs, and compare it with the 3% molar in yttria (3Y-TZP). Several patterns consisting in parallel tracks below 10 µm depth were investigated. The results evidenced a heat affected zone (HAZ) limited to ⁓1–2 µm with microcracking and directional recrystallization, which was larger in 8YSZ than in 3Y-TZP. However, the mechanical response near the HAZ and chemical composition at the machined surface was not significantly changed.     

Fabrication of micro-scale textured grooves on green ZrO2 ceramics by pulsed laser ablation

The green ZrO₂ ceramics were fabricated by cold isostatic pressing. Pulsed laser ablation with a wavelength of 1064 nm was performed to fabricate micro-scale textured grooves on the surface of green ZrO₂ ceramics. The influence of laser parameters on surface quality was studied. The heat-affected zone around the machined grooves and micromorphology of laser-irradiated surface were investigated. Results showed that micro-scale textured grooves with a width of 30–50 µm and a depth of 15–50 µm on the green ZrO₂ ceramic surfaces were successfully fabricated by pulsed laser ablation. The laser parameters had a profound influence on the surface quality of micro-scale textured grooves. Better surface quality could be obtained with frequency below 40 Hz, power below 6 W, and scanning velocity above 200 mm/s. A sintering layer was found on the laser-irradiated surfaces when frequency was above 60 Hz, power was above 10 W, and scanning velocity was below 150 mm/s. Analysis of this sintering layer revealed clear melting and resolidification of ZrO₂ particles.     

Optimization,formation, and evolution of the photoinduced curing gradients and in-situ lamellar gaps in additive manufacturing of ZrO2 ceramics: From curing to sintering behaviors

The photoinduced curing gradients and in-situ lamellar gaps were related to the three-dimensional curing mechanism of stereolithography and researched in this work. Based on the photocuring mathematic theories presented by our team, photoinduced curing gradients were influenced by printing parameters such as laser power and frequency, scanning angle, and printing thickness. Among these curing gradients, the inadequate curing area was the main factor affecting the surface quality and physical properties of ceramics. The in-situ lamellar gaps were derived from the inadequate curing area. The lamellar gap size of green bodies was enlarged in brown bodies (sintering at 1170 ℃) and shrunk in sintered bodies (sintering at 1500 ℃). The correlations between the surface quality and printing parameters were clarified by establishing mathematical formulations of the lamellar gap size. Furthermore, anisotropic volume shrinkage, anisotropic surface topography, and density and hardness evolutions were associated with the wrapped particle density differences induced by the photoinduced curing gradients. Finally, the optimal printing parameters (75% of laser power, laser frequency of 150 kHz, scanning angle of 90º, printing thickness of 25 µm, and scanning speed of 2.5 m/s) were obtained by the orthogonal experimental analysis. This study provides a distinct understanding of the high-quality forming and extreme-performance manufacturing of stereolithography.     

Tribological characterization of NCD in physiological fluids

NCD films deposited on silicon nitride (Si₃N₄) ceramic substrates by hot-filament chemical vapour deposition (HFCVD) technique were biotribologically assessed under lubrication of Hank's balanced salt solution (HBSS) and dilute fetal bovine serum (FBS), using a pin-on-flat test configuration. The reciprocating tests were conducted under an applied load of 45 N during 500,000 cycles using a NCD coated Si₃N₄ biocompatible ceramic substrates with two different surface preparations: i) polished (P) and ii) polished and plasma etched (PE). Friction coefficient values of 0.02 and 0.12 were measured for the P samples under HBSS and FBS lubrication, respectively. PE samples showed increased adhesion relatively to P ones and withstood 6 km of sliding distance without any evidence of film fracture but with friction coefficients of 0.06 for HBSS and 0.10 for FBS experiments. Evidences of protein attachment and salt deposition were found, being the responsible for the enhancement of friction under FBS relatively to HBSS. The wear rates measured for the NCD films are in the range of ~10^− 9–10^− 8 mm³·N^− 1m^− 1, values that are similar to the best values found for ceramic-on-ceramic combinations.     

Influence of Different Solvents on the Morphology of the Hybrid Layer Using Two-Step Etch-and-Rinse Adhesives

To evaluate if different etch-and-rinse adhesive solvents influenced the hybrid layer's morphology.

Four one-bottle etch-and-rinse adhesive systems containing different solvents—Group A: Scotchbond 1XT?– 3 M ESPE, Group B: XP – Bond? – Dentsply, Group C: Prime&;Bond NT® – Dentply, and Group D: One Coat Bond® - Coltène Whaledent—were applied onto 32 dentin discs which were thermocycled, prepared, and examined using field-emission scanning electron microscopy. Micrographs were scanned and the data were processed using Statistical Package for Social Sciences. The mean value and standard deviation were calculated and the Anova Multivariant Test was used.

The hybrid layer thickness average found was 3.23 µm (±0.53) in Group A, 3.13 µm (±0.73) in Group B, 2.53 µm (±0.50) in Group C, and 1.84 µm (±0.27) in Group D. Prime&;Bond NT® presented a more inconsistent hybrid layer.

The solvent seems to play a significant role in hybrid layer structure and thickness.     

Field emission from nanodiamond grown with ‘ridge’ type geometrically enhanced features

Cold cathodes for vacuum field emission have many interesting applications including display devices. Chemical vapor deposited (CVD) diamond has long been considered such a candidate. A nanodiamond film with ridge features was grown on a highly doped (resistivity = 0.0035 Ω-cm) n-type silicon substrate by plasma enhanced CVD process using a H2/CH4/N2 gas mixture. The overall planar nanodiamond film was characterized for vacuum field emission in diode configuration. A low turn on field of ~ 2.3 V/µm at 1 µA was observed. The cathode was able to produce 150 µA at a field of 6.6 V/µm. This field emission behavior can be attributed to a combination of high sp2 content, higher electrical conductivity by nitrogen incorporation in the nanodiamond film and a high geometrical field enhancement factor because of the sharp ‘ridge’ like features on the surface.     

Influence of surface treatments on topography and bond strength of densely-sintered zirconium-oxide ceramic

The aim of this study was to evaluate the effect of surface treatments on the roughness and bond strength of dental materials containing MDP to zirconium oxide ceramic. Forty square-shaped zirconium-oxide ceramic blocks (Lava Zirconia, 3M-ESPE) were treated as follows: (CT) polished only; (SB) sandblasting (110 µm aluminum oxide particles) or (SC) silica coating (110 µm particles). Roughness of treated surface was measured using a profilometer (Ra) and by atomic force microscope (AFM). Two resin luting agents were used after silane application: self-adhesive (Rely X U200, 3M-ESPE) and dual cure (Rely X Ultimate, 3M-ESPE). The samples were submitted to microshear bond strength test. The failure analysis was performed. Data were submitted to ANOVA and Tukey test (α=0.05). Bond strength results ranged from 20.44 (CT+Ultimate) to 34.37 MPa (SC+U200) after 24 h and from 12.03 (CT+Ultimate) to 27.44 MPa (SC+U200) after 12 months of storage with SC statistically superior to the other treatments. Mean values of roughness varied from 0.07 (CT) to 0.85 µm (SC). The both resin luting agents showed similar results to all surface treatment groups. Silica coating provided the best treatment of the ceramic surface.     

DC and RF performance of surface channel MESFETs on H-terminated polycrystalline diamond

DC and RF performance of submicron gate-length metal–semiconductor field effect transistors (MESFETs) fabricated on hydrogen-terminated polycrystalline diamond is investigated in detail for different material electronic quality (grain size in the range 100–200 µm) and device geometry (drain-source channel length in the range 1–3 µm). DC characteristics appear almost independent of both properties, giving maximum drain-source current values in the range 120–140 mA/mm in MESFETs having same gate length (0.2 µm) and gate width (25 µm). The layer properties underneath the hydrogenated surface seem then to affect the DC behaviour to a lesser extent when the same hydrogenation procedure is used. At variance, the electronic quality of diamond layers employed for MESFETs realization largely affects the RF performance, resulting into a low oscillation frequency f_max for a MESFET realized by a self-aligned process (1 µm drain-source channel length) onto low quality diamond polycrystalline film. Such a performance improves to f_max = 35 GHz for devices realized onto large grain polycrystalline diamond, although fabricated without self-aligned gate procedure (3 µm drain-source channel length). These findings are discussed in terms of different roles played by surface hydrogenation, device geometry detail and electronic quality of the polycrystalline diamond substrate for MESFET realization.     

Centimeter-scale low-damage micromachining on single-crystal 4H–SiC substrates using a femtosecond laser with square-shaped Flat-Top focus spots

Femtosecond (fs) lasers have been proved to be reliable tools for high-precision and high-quality micromachining of ceramic materials. Nevertheless, fs laser processing using a single-mode beam with a Gaussian intensity distribution is difficult to obtain large-area flat and uniform processed surfaces. In this study, we utilize a customized diffractive optical element (DOE) to redistribute the laser pulse energy from Gaussian to square-shaped Flat-Top profile to realize centimeter-scale low-damage micromachining on single-crystal 4H–SiC substrates. We systematically investigated the effects of processing parameters on the changes in surface morphology and composition, and an optimal processing strategy was provided. Mechanisms of the formation of surface nanoparticles and the removal of surface micro-burrs were discussed. We also examined the distribution of subsurface defects caused by fs laser processing by removing a thin surface layer with a certain depth through chemical mechanical polishing (CMP). Our results show that laser-induced periodic surface structures (LIPSSs) covered by fine SiO₂ nanoparticles form on the fs laser-processed areas. Under optimal parameters, the redeposition of SiO₂ nanoparticles can be minimized, and the surface roughness Sa of processed areas reaches 120 ± 8 nm after the removal of a 10 μm thick surface layer. After the laser processing, micro-burrs on original surfaces are effectively removed, and thus the average profile roughness Rz of 2 mm long surface profiles decreases from 920 ± 120 nm to 286 ± 90 nm. No visible micro-pits can be found after removing ~1 μm thick surface layer from the laser-processed substrates.     

Ultra-fast,selective, non-melting,laser sintering of alumina with anisotropic and size-suppressed grains

In this paper, we report an ultra-fast sintering phenomenon of alumina achieved by the scanning laser irradiation method. Using CO₂ laser irradiation, we found that micrometer-sized alumina powder (d₅₀ = 1.2 µm) can be sintered close to full density within a few tens of seconds. The microstructure of laser-sintered alumina was different from that of the furnace-sintered alumina. The relative density and grain size of the laser-sintered alumina gradually decreased from the center of the laser beam to the edge. Anisotropy of the grain size was measured along and perpendicular to the scanning direction. This anisotropy decreased as the scanning speed decreased from 0.1 mm/s to 0.01 mm/s. The sintering master curve of grain size versus relative density, which reflects the sintering mechanism, was found to be affected by the laser scanning speed. When the laser scanning speed was 0.1 mm/s, grain size suppression was found for the almost fully dense alumina. However, at lower scanning speed (e.g., 0.01 mm/s), there was significant grain growth in the regions where the relative density was greater than 90%. These results clearly indicate that alumina can be sintered, in the solid-state, to a high density in a short time using scanning laser and the microstructure is different from the furnace-sintered alumina.     

Growth kinetics of WC-Fe layer formed at the surface iron during solid-phase diffusion

Tungsten carbide-reinforced iron-based surface composites were prepared via in situ solid-phase diffusion method; the variables included three temperatures (1085, 1100, and 1125 °C) and four heat treatment times (15, 45, 75, and 105 min). The samples were examined by X-ray diffraction, scanning electron microscopy, and Vickers hardness test. Results show that the tungsten carbide-reinforced iron-based surface composites consist of WC, α-Fe, W, and iron carbide phases, and the thickness of the WC-Fe layer ranges from 20.57±1.24 µm to 63.27±2.02 µm at 1085 °C. Furthermore, the maximum microhardness value of the WC-Fe layer at 1085 °C for 15 min is 2169 HV_0.1, whereas that of the iron matrix is 239 HV_0.1; such values demonstrate that the hardness of the composites are markedly enhanced. The kinetic of WC-Fe layer was analyzed by measuring the depth of pure WC layer as a function of heat treatment time and temperature. The results show a parabolic relationship between the thickness of pure WC layer and heat treatment time, and the activation energy for the pure WC layer was estimated to be 184.06 kJ mol⁻¹.     

Study on surface quality,precision and mechanical properties of 3D printed ZrO2 ceramic components by laser scanning stereolithography

Zirconia (ZrO₂) ceramic bars with three different printing sizes were fabricated by a stereolithographic (SLA) 3D-printing process and subsequent sintering. An anisotropic character of the ceramics surface quality was observed. The surface roughness of the horizontal surface was below 0.41 µm, whereas it reached 1.07 µm along the fabrication direction on the vertical surface. The warpage and flatness were utilized to measure the dimensional accuracy of the 3D printed ZrO₂. Furthermore, it was evaluated that the warpage and flatness were below 40 µm and 27 µm, respectively, even if the printed size of ceramic bar reached 3 mm × 4 mm × 80 mm. In addition, the flexural strength, the fracture toughness, the hardness and the density of ZrO₂ ceramics can reach to 1154 ± 182 MPa, 6.37 ± 0.25 MPa m^1/2, 13.90 ± 0.62 GPa and up to 99.3%, respectively. Moreover, the effects of scanning paths and printing size on properties of the sintered ZrO₂ samples were analyzed. The anisotropic character of surface quality was related to the various scanning paths. The warpage and flatness of 3D printed ZrO₂ bars were apparently affected by the various printed sizes. Also, the effects of special microstructure on the mechanical properties of sintered ZrO₂ samples were investigated.     

Effect of an Er,Cr:YSGG laser preparation on dentin bond strength of a universal adhesive

The aim of this study was to evaluate the bond strength of a universal adhesive system to dentin prepared with SiC paper or an Er,Cr:YSGG laser using different bonding strategies (etch-and-rinse versus self-etch mode). Ninety-six extracted caries-free, sound human molars were used. The teeth were longitudinally sectioned in the mesiodistal direction and were wet polished with 600-grit SiC paper to obtain a standardized flat dentin surface. All prepared teeth were randomly divided into two groups, according to the surface preparation method: GroupI:an erbium, chromium:yttrium,scandium, gallium, garnet laser; Group II: silicon carbide paper[SiC] (n = 48). Each group was then assigned into three subgroups according to the universal adhesive’s (Single Bond Universal) bonding strategies: (a) etch-and-rinse mode with phosphoric acid, (b) etch-and-rinse mode with a laser, (c) self-etch mode (n = 16). For surface preparation, the Er,Cr:YSGG laser was used at 3 W, 30 Hz with 140 μs pulse duration for 25 s. For etching mode, the laser was used at 1.5 W (60% air, 70% water). Cylinders of composite were fabricated on the bonding area and shear bond strength was determined using a universal testing machine. Failure modes were evaluated using a stereomicroscope. The data were analyzed using two-way ANOVA followed by the Bonferroni test (p < 0.05). Bonding strategies showed statistically significant differences in both the SiC-and laser-prepared groups (p < 0.05).Universal adhesive used in etch-and-rinse mode with acid showed significantly higher bond strength values than in self-etch mode (p < 0.05). The bond strength values did not differ according to the surface preparation method (p > 0.05). Irrespective of preparation method, using universal adhesive in etch-and-rinse mode with acid might improve dentin bond strength. Laser preparation did not affect the bond strength of the universal adhesive tested.     

Laser surface modification of porous yttria stabilized zirconia against CMAS degradation

Here, we present a new combined freeze-casting and laser processing method for the design of yttria-stabilized zirconia (YSZ) based thermal-barrier coatings. YSZ ceramics with unidirectionally-aligned pore channels were created using the freeze-casting method. After sintering, top view and cross-sectional scanning electron microscopy (SEM) revealed the structural features of the preform, which exhibits a 74 ± 2% volume fraction of porosity and an average pore channel size of 30 ± 3 μm. The measured thermal conductivity of this porous structure was 0.27 ± 0.02 W/(m K), which is eight times lower than that of reported values for dense YSZ. Though high porosity is beneficial both from a structural and thermal response perspective, the open porosity could potentially be an issue from an application stand-point when evaluating the resistance of materials to calcium–magnesium–aluminum–silicon oxide (CMAS) attack. CMAS attack, which can originate from deposits of molten sand, ash, and dust, is one of the major causes of thermal barrier coating failure. Therefore, the surface of the porous samples was modified using a laser process to create a barrier to CMAS infiltration. SEM micrographs aided in determining the optimum laser parameters required to fully seal the surface using a laser treatment. The performance of the original porous and surface-modified YSZ was compared by conducting CMAS infiltration studies. Laser modification was shown to be a viable technique to significantly reduce CMAS infiltration in porous thermal barrier coatings.     

Femtosecond laser micromachining in combination with ICP etching for 4H–SiC pressure sensor membranes

4H–SiC is one of the most promising materials for pressure sensing in harsh environments. A Yb:KGW femtosecond laser was employed to fabricate 4H–SiC sensor membranes with size of Φ1200 × 80 μm. The optimal parameter combination under 15 μJ single pulse energy was obtained with the laps of 16, the scanning speed of 130 mm/s, the scanning line interval of 2 μm and the repetition rate of 100 kHz. High size accuracy (±1%) and steep sidewall (87.4°) were achieved. Wet cleaning and inductively coupled plasma (ICP) etching can obviously improve the membrane bottom surface morphology. The surface roughness Ra in X direction was reduced from 0.82 μm to 0.15 μm, and that in Y direction was reduced from 1.32 μm to 0.16 μm. Pinhole defect was related to the nonuniform distribution of laser fluence. This defect can be avoided by reducing the laser spot overlap ratio. Energy-Dispersive X-ray Spectroscopy (EDS) and Raman spectrum were adopted to analyse the changes of material properties after laser processing. The analysis indicated that the crystal properties of the membrane bottom and the thin epitaxial layers on the front side of membrane are not damaged by the integrating micromachining. The results indicate the potential of utilizing the femtosecond laser combined with ICP etching to fabricate 4H–SiC sensor membranes.     

The effects of laser patterning 10CeTZP-Al2O3 nanocomposite disc surfaces: Osseous differentiation and cellular arrangement in vitro

Customized square grid arrangements of different groove depths (1.0, 1.5 and 3.0?µm) and separations (10 and 30?µm) were successfully laser patterned, using a nanosecond pulsed fibre laser, on the surface of 10?mol% ceria-stabilized zirconia and alumina (10CeTZP-Al₂O₃) nanocomposite discs (diameter: 10?mm; thickness: 1.5?mm). The patterned surfaces and the in vitro biological response of osteoblasts (SAOS-2) towards them were thoroughly analysed. In terms of composition, the laser treatment was found to cause superficial monoclinic-tetragonal zirconia phase transformation and alumina evaporation. In vitro, the most effective grid configuration for osseous differentiation was found to be 1.5?µm groove depth and 10?µm groove separation, and confocal microscopy revealed that the cells show a tendency to be sorted as groove depth increases. It is thought that custom-made patterns could be produced to guide cell attachment in vivo, which could favour implant integration and reduce healing time.     

Bond strengths of bulk-fill resin composite repairs: effect of different surface treatment protocols in vitro

To evaluate the effect of different surface treatment protocols on the microtensile bond strength (μTBS) of bulk-fill resin composite repairs. Thirty-five bulk-fill resin composite samples (Filtek Bulk Fill) were prepared (5 × 5 × 5 mm) and aged by thermocycling (X5000). Samples were randomly divided into five groups (n = 7): a control (no treatment) and four surface treatment groups (Single Bond Universal [SBU]; phosphoric acid (37%) + SBU; Er,Cr:YSGG laser + SBU; aluminum oxide sandblasting + SBU). Filtek Ultimate Universal composite was used as a repair material. After storage for 24 h in distilled water (37 °C), sticks were obtained and subjected to a μTBS test. The data (MPa) were analyzed by one-way ANOVA with a post hoc test (α = 0.05). Failure mode was evaluated using a light microscope (10×). There were significant differences between the groups (p < 0.05). The lowest bond strength values were obtained in the control group (p < 0.05). No significant difference was observed between Group II (universal adhesive) and Group III (acid etch + universal adhesive) (p > 0.05). The bond strength of Group II was significantly lower than that of the other surface treatment groups (p < 0.05). While Group III showed significantly lower values than those of the laser treatment group (Group IV), similar values were obtained with Al₂O₃ sandblasting group (Group V). The highest repair bond strength was obtained in Group IV (p < 0.05) which was not significantly different from the Al₂O₃ sandblasting group (p > 0.05). The predominant failure mode was adhesive. Treatment of aged bulk-fill resin composite surfaces with laser and Al₂O₃ sandblasting provided higher repair bond strength values.     

Fabrication of Gentamicin Sulfate-Loaded 3D-Printed Polyvinyl Alcohol/Sodium Alginate/Gelatin-Methacryloyl Hybrid Scaffolds for Skin Tissue Replacement

3D-printed scaffolds can better mimic the function of human skin, both biologically and mechanically. Within the scope of this study, the effect of the addition of different amounts (10, 15, 20 mg) of gentamicin sulfate (GS) to a 10 mL solution of natural and synthetic polymers is investigated. Sodium alginate (SA), gelatin-methacryloyl (GelMA), and polyvinyl alcohol (PVA) are chosen as bioactive materials. The surface morphology and pore structures are visualized by scanning electron microscopy (SEM). According to the results, it is observed that the pore sizes of all scaffolds are smaller than 270 µm, the lowest value (130 µm) is obtained in the scaffold loaded with 15 mg GS, and it also has the highest tensile strength value (12.5 ± 7.6 MPa). Similarly, it is observed that the tensile strength (9.7 ± 4.5 MPa) is high in scaffold loaded with 20 mg GS. The biocompatibility test is performed with fibroblast cells, and the results show that the scaffolds are biocompatible with cells. The antibacterial test is carried out against the S.aureous and E. coli and the results indicate that all GS-loaded scaffolds demonstrate antibacterial activity.